For high strength polyethylene fibers, there have been disclosed, for example, in JP-B 60-47922, high strength, high modulus fibers produced by the “gel spinning method” using ultrahigh molecular weight polyethylene as the base material. These high strength polyethylene fibers have already been widely used in various fields for industry, for example, as the ropes or nets for industrial or private use; as high performance textiles such as ballistic materials or items, or protective gloves; or as the geo-textiles or working nets in the filed of civil engineering and architecture.
In recent years, these high strength polyethylene fibers have been required to have further improved performance, particularly durability, for example, mechanical durability over a long period or adaptability under severe conditions in use. Even textiles such as sportswears, or fishing lines have also been required to have durability when used for a long period. In addition, reinforcing sheets or strands to provide earthquake resistance have been required to have durability, particularly flexural fatigue resistance or abrasion resistance, such that when wound around pillars or other parts they cause no occurrence of fiber breaking at the corners.
The high strength polyethylene fibers have excellent tensile strength and excellent Young's modulus indeed, but on the other hand, the structure of their highly-oriented molecular chains is responsible for the drawback that they have poor durability, particularly poor flexural fatigue resistance and poor abrasion resistance, for example, as compared with polyesters or nylons for ordinary garments. Such drawback has become some obstacle to the wide application of high strength polyethylene fibers in various fields for industry.
Further, many attempts have been made to use high strength polyethylene fibers in the chemical processes, for example, application to non-woven fabrics such as chemical filters or battery cell separators, because of their excellent resistance to chemicals, light and weather or to apply high strength polyethylene fibers to reinforcing fibers for concrete or cement, because there has been a demand for fiber reinforced concrete products having high crack resistance and high toughness, as well as excellent impact resistance and excellent long-term durability, since accidents were caused by wall materials coming off or falling from the surface of railroad tunnels or bridges.
However, when chopped fibers or staples are produced by cutting the conventional high strength polyethylene fibers, fibrillation of the fibers or their high surface hardness is responsible for the drawback that these fibers get stuck together by pressure to form a bundle of fibers, lacking in dispersibility. Further, when they are used as the reinforcing fibers for concrete or cement, their dispersibility in the cement matrix becomes deteriorated by flexure or entanglement of the fibers. For this reason, various treatments have been needed, for example, premixing with cement, hydrophilicity-providing treatment using metal oxides, or binding with resins.